The invention relates to a new agent and consequently a better method for the removal of photoresist and stripper residues from semiconductor substrates.
At the present time the manufacture of semiconductor components and integrated circuits is carried out virtually exclusively using photolithographic processes. For the purposes of patterning, the semiconductor substrates are coated with photoresists (positive or negative photoresists) and photoresist relief patterns are generated on them by imagewise exposure and subsequent development. These patterns then act as a mask for the actual patterning processes such as etching, doping, coating with metals, other semiconductor materials or also insulating materials. After these processes, the photoresist masks have to be removed again. For this purpose special stripping agents known as strippers in the relevant linguistic usage are used. These are normally mixtures of organic solvents which contain in addition surface-active substances, in general mixtures of alkylbenzenesulphonic acids.
To remove the photoresist coatings, the semiconductor materials (generally these are silicon discs, so-called wafers) are immersed in a stripper bath of this type which, to increase its effectiveness, is normally heated to temperatures between 80.degree. and 150.degree. C., usually around 100.degree. C. When this is done, the photoresist is stripped off after a more or less short time.
After the stripping off of the resist, it is necessary for still-adhering residues of stripper as well as any incompletely removed photoresist residues or degradation products thereof to be removed from the semiconductor materials in order to obtain a completely clean surface which meets the high purity requirements of semiconductor technology. This can be done, for example, in one or more intermediate baths, for example in organic solvents such as acetone, alcohols or xylene and in a subsequent post-rinse with de-ionized water.
Simply for reasons of process simplification and of making savings, but also for reasons of safety at the place of work and protection of the environment, attempts are being made to avoid the intermediate bath step using organic solvents.
Accordingly it has become the practice to rinse off the semiconductor substrate directly with water immediately after removal from the stripper bath. However, if a water encumbered with stripper residues is not first immersed in an organic solvent after the stripping process but directly into a stationary water bath, then a deposit is formed as a result of alkylbenzenesulphonic acid precipitating. The latter can be removed with water and this takes place the more rapidly, the more vigorous the motion of the water bath. More beneficial is rinsing off under running water, in which case the latter should if possible be under slight pressure. The rinsing off of stripper residues with water immediately after the stripping process assumes, however, that the photoresist has been completely stripped off during stripping and that the wafer is only encumbered with stripper residues after removal from the bath but not with photoresist residues. Experience shows that this is in general the case in the stripping of negative photoresists. In the case of negative photoresists, which are predominantly based on cyclized rubber materials and light-sensitive bisazido compounds, a post-rinse with water after the stripping process is consequently possible.
The circumstances prove to be more difficult in the case of positive photoresist materials. Materials of this type are usually based on formaldehyde-phenol condensates of the novolak resin type and light-sensitive quinonediazidesulphonyl derivatives. The photoresists usually require a heat-treatment in the preparation of the resist coating and after the photopatterning to increase the adhesion to the substrate. In this process the temperatures are between 100.degree. and 200.degree. C., usually between 140.degree. and 180.degree. C. In this connection the improvement in the properties is decisively dependent on the applied temperature. In the subsequent plasma etching or ion implantation processes the coatings are also exposed to fairly high temperatures. Positive resists of this type cannot be completely removed from the substrate without difficulty by the stripper solution, in particular if treated at high temperature. When freshly stripped wafers are immersed in water, unremoved laquer residues form, together with precipitating stripper components and stripped photoresist components, an essentially insoluble covering on the semiconductor surface. This covering can only be removed with difficulty or not at all even with running water and under pressure so that completely clean surfaces are only obtainable by including an intermediate bath containing organic solvents beforehand.